In this paper, numerical simulation was conducted using Fire Dynamics Simulator (FDS) to study bifurcation flow of smoke in tunnel with longitudinal ventilation. Through analyzing the effect of longitudinal ventilation velocity on smoke flow field, temperature distribution within smoke layer and height of smoke layer, generation mechanism of smoke bifurcation flow was revealed. Longitudinal ventilation would increase entrainment of smoke with air and dilute the smoke at the center of tunnel ceiling, and would enhance inertial force of horizontal flow and weaken driving force of reverse flow of ceiling jet by sidewall. A low temperature region below tunnel ceiling was formed when longitudinal ventilation exceeded a critical value. There was little smoke in the low temperature region, so smoke bifurcation flow was occurred. The occurrence of the bifurcation flow mainly depends on the horizontal inertial force and the driving force. (C) 2013 Elsevier Ltd. All rights reserved.